Fast poloidal rotation and improved confinement

Recently it has been shown on many tokamaks that a transition into regimes with improved confinement is accompanied by an increase in the poloidal rotation velocity Vp at the edge of a tokamak. This is also borne out by experiments on the tokamak ''TUMAN 3'', where large poloidal rotation velocities much in excess of the poloidal sound speed ΘCS (Θ=Bp/B and CS=(T/mi)1/2) were induced by means of the externally biased electrode. Here, the temporal and the spatial relaxation of an arbitrary poloidal rotation velocity is considered. It is shown that extending the neoclassical expressions for the parallel viscosity and the inertia for the plateau regime into Vp>ΘCS yields qualitatively different results compared to Vp<<ΘCS. The parallel viscosity and the inertia turn out to be the grossly nonlinear functions of the Vp. Moreover for Vp≥ΘCS, the time scale for the relaxation of Vp is strongly enhanced versus Vp<<ΘCS. Furthermore the quasi-stationary state does not emerge at all (the torque provided by the inertia can not be damped by viscosity) within the framework of the neoclassical theory provided that Vp>>ΘCS. Therefore the inertia and the viscosity driven by the turbulence are invoked resulting in the quasi-stationary profile of the poloidal rotation velocity. Thus our model accounts for fluxes driven by a turbulence and therefore is more general than the neoclassical theory. Resulting profiles of Vp acquire the large flow shear, beneficial for suppression of edge turbulence by decorrelation. (author) 8 refs